The present invention relates to a spectroscopic optical device, especially to a reflection type diffraction optical device used widely in an optical technology field such as an optical sensor, an information recording device, an optical measuring device, and so on.
Diffraction gratings are used popularly as spectroscopic optical devices or optical demultiplexing devices. Among these diffraction gratings, a reflection type diffraction grating using diffracted light as reflected light is, in most cases, configured in such a manner that a metal film is formed as a reflection film on a surface of a member having grooves formed periodically so as to improve reflection efficiency of the reflection type diffraction grating. A high-reflectance material such as Al, Au, or the like, is used as the metal film provided on the surface of the periodically grooved member.
If such reflection metal films are used in a state of being exposed to an environment, weather resistance and abrasion resistance of these reflection metal films may be insufficient. Particularly it is known that an Al film is inferior in oxidation resistance. An Au film is insufficient in mechanical strength. In order to improve durability of such a reflection metal film, generally, the planar metal film is coated with a transparent dielectric film or the like. It is known that oxidation resistance particularly in the case of a reflection Al film is improved when a magnesium fluoride (MgF2) film is used as a protective film of the reflection Al film. It is also known that the MgF2 film has an effect in improving reflectance in a near-ultraviolet region.
It is however known that a reflection type diffraction grating having a groove period in a range of from 0.1 to 10 times as large as the wavelength of incident light has its diffracted light intensity varying in dependence on polarization of incident light (for example, see O plus E, Vol.21, No.5, p.511 (1999)). A polarization separating device positively using this characteristic for separating polarized light has been proposed (for example, see Optronics, No.8, p.112 (1996)).
On the other hand, when such a diffraction optical device is used as a demultiplexing device in the field of optical communication or the like, it is necessary to keep the intensity of diffracted light constant regardless of the state of polarization of incident light. For example, in wavelength-multiplex communication, light with a large number of wavelengths which is transferred by one optical fiber needs to be spectrally distributed by a diffraction grating in order to read information of individual wavelengths. On this occasion, the state of polarization of light emitted through the optical fiber is not controlled. Hence, when the light is made incident on the diffraction grating directly, the intensity of diffracted light depends on the state of polarization. There arises a problem that information processing of such light is made complex.
Also when a reflection type diffraction optical device is applied to the case where light from a laser light source is condensed and irradiates a fine roughness structure of the diffraction optical device to read out the shape of the fine roughness structure, sensitivity of the reflection type diffraction optical device is lowered in accordance with the state of polarization of the laser light source so that signal analysis is made complex.
To control the polarization of light incident on such a diffraction optical device, a method of inserting a polarizer, a filter or the like in an optical system has been proposed. In the proposed method, however, insertion loss is produced because of the inserted device as well as the optical system is made complex because of increase in the number of devices. There is a problem that the intensity of light is lowered.